Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Which Morelia audios are available?

  1. Jul 15, 2007 #1


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    The Loops 07 site has been gradually adding mp3 files for the plenary and parallel talks, the most recent additon was 13 July. Here are some plenary talks that have audios---which I've listened to while scrolling thru the slides.
    In this first batch, I had some idea already what to expect from the talks by Ashtekar, Rovelli, and Thiemann. So although the talks were clear, organized, informative, I was not bowled over. On the other hand I want to say YOU HAVE TO WATCH the talks by ORITI, REUTER, and RIDEOUT. For me, they had a surprise factor that made them extra impressive.
    Oriti has important new results in Group Field Theory (background independent field including gravity translated to live on a cartesian product of groups).
    Reuter is a highly credible contrarian, or so I found.
    Rideout and co-workers apply supercomputers to quantum gravity (see also supercomputing reports in parallel sessions by Christensen, Cherrington, Khavkine)

    Plenary talks

    Abhay Ashtekar: LQG: Lessons from models · slides (pdf) · audio (mp3)

    In the last couple of years, several simple but physically interesting models were solved. Solutions led to concrete and detailed realizations of a number of ideas that have been heuristically expected for decades. There were also some surprises. These analyses suggest viewpoints and strategies for full quantum gravity. I will summarize some of them in broad terms.

    Daniele Oriti: Group field theory: spacetime from quantum discreteness to an emergent continuum · slides (pdf) · audio (mp3)

    Group field theories are non-local quantum field theories on group manifolds, and a generalization of matrix models. Having been first introduced in the context of simplicial quantum gravity, have gained attention as being potentially of much interest in the context of loop quantum gravity and spin foam models. After a brief introduction to the group field theory formalism, I review some of the results already obtained in this approach. I will then try to offer a new perspective on how group field theories should be interpreted and used towards a complete theory of quantum gravity. In particular, I will argue that group field theories can represent on the one hand a common unifying framework for loop quantum gravity, spin foam models and simplicial approaches, like quantum Regge calculus and dynamical triangulations, and on the other hand a consistent microscopic description of spacetime considered as a condensed matter system. From this, a novel approach to the issues of the emergence of the continuum and of General Relativity as an effective description of spacetime, in this approximation, is proposed. Finally, I will briefly report on some recent results and work in progress inspired by and supporting this new perspective.

    Martin Reuter: Asymptotically safe quantum gravity and cosmology · slides (pdf) · audio (mp3)

    The basic ideas and main results of the asymptotic safety scenario in Quantum Einstein Gravity (QEG) are reviewed and possible implications for the cosmology of the early universe are discussed.

    David Rideout: Can the supercomputer provide new insights into quantum gravity? · slides (pdf) · audio (mp3)

    Machine computation is a relatively unutilized tool in quantum gravity, in part because of the enormous scale of the problems which arise, and the corresponding substantial initial investment which must be made to write code for supercomputers. However, the advent of computational frameworks, such as Cactus, is changing this situation, by drastically decreasing the time required to develop code for supercomputers. I will briefly describe the Cactus framework, and present three insights that have arisen from its use. One is with regard to how entropy bounds may arise from discrete gravity, a second regarding how continuum topology may emerge from an underlying causal set, and third how the nature of the spectrum of the Ashtekar-Lewandowski volume operator of Loop Quantum Gravity depends crucially on the nature of the embedding of a spin network vertex.

    Carlo Rovelli: The LQG vertex · slides (pdf) · slides (key.zip) · audio (mp3)

    It has recently been possible to begin the computation of n-points functions in loop quantum gravity. I review the basic ideas and the present state of these calculations. The Barrett-Crane vertex appears to yield some n-point functions with the correct low-energy limit, but there are also indications of a wrong behavior. The problem can be traced to the way intertwiner quantum numbers are treated in the Barrett-Crane model. This is also the source of the general discrepancy between the Barrett-Crane vertex and LQG. I present a new vertex for loop quantum gravity, introduced in collaboration with Jonathan Engle and Roberto Pereira, which may correct the problem and is fully consistent with the LQG kinematics.

    Thomas Thiemann: Loop Quantum Gravity (LQG): From secured land to unknown territory · slides (pdf) · audio (mp3)

    We describe the conceptual and mathematical setup of Loop Quantum Gravity (LQG).

    Last modified 13 July 2007.
    Here are some talks for which slides, but not yet audio files, are available---or for which we don't yet have slides---or which I simply haven't yet had time to take in.

    Jan Ambjørn: 4d quantum gravity as a sum over histories · audio (mp3)

    In this plenary talk I will review the attempts to formulate 4d quantum gravity as a sum over histories in such a way that computer simulations can be performed. I will report on computer simulations of a quantum universe with a positive cosmological constant as well as a quantum universe where test matter is included.

    Martin Bojowald: Loop quantum gravity and effective theory · slides (pdf)

    Several models have been defined to study the broad framework of loop quantum gravity. We present a common perspective for cosmology where configurations are close to being isotropic. Effective techniques are then used to illustrate important features of the semiclassical limit and to show examples of correction terms to Einstein's equation. Applications include corrections to the Newton potential and to cosmological perturbation equations relevant for the CMB power spectrum.

    Sabine Hossenfelder: Phenomenological Quantum Gravity · slides (pdf)

    The search for a satisfying theory that unifies general relativity with quantum field theory is surely one of the major tasks for physicists in the 21st century. During the last decade, the phenomenology of quantum gravity and string theory has been examined from various points of view, opening new perspectives and testable predictions. I will give a short introduction into these effective models which allow to extend the standard model and include the expected effects of the underlying fundamental theory. I will talk about models with extra dimensions, models with a minimal length scale and those with a deformation of Lorentz invariance. The focus is on observable consequences, such as black hole and graviton production and modifications of standard-model cross-sections.

    Fotini Markopoulou: Quantum gravity and emergent locality · slides (pdf)

    An important aspect of the desired low energy limit of a background independent quantum theory of gravity is to show how locality arises in the emergent low energy theory. We discuss why we should not expect the high energy theory to have our geometric notions of locality. We present two models of a high energy breakdown of locality: a disordered locality perturbation of a flat geometry and a fully pre-geometric system with no notion of locality which has a local ground state.

    Alejandro Perez: Regulator dependence in quantum gravity and non perturbative renormalizability: possible new perspectives · slides (pdf) · audio (mp3)

    Topological field theories are simple examples of background independent field theories which are non perturbatively renormalizable in the sense that regularization ambiguities have no effect on physical quantities. Three dimensional vacuum general relativity coupled to point particles is an important example of such theory. The corresponding generalization to higher dimensions has been studied recently. In four dimensions, one-dimensional extended objects (strings) are the natural form of matter that couples to four dimensional BF theory. After briefly reviewing these models, we will show how these topological theories of extended objects can accommodate physically interesting degrees of freedom maintaining their topological nature. We will propose a way by which topological theories of 2d world-sheet matter (of the kind presented in the first part of the talk) might be used to construct background independent quantum field theories with local degrees of freedom and no regulator dependence.

    Lee Smolin: Chiral excitations of quantum geometry as elementary particles · slides (pdf) · slides (ppt)

    Rafael Sorkin: Quantum reality and anhomomorphic logic · audio (mp3)

    Histories-based forms of quantum mechanics seem better suited to the needs of quantum gravity than the more familiar alternative based on state-vectors and selfadjoint operators. Such a re-casting of the theory opens up the possibility to dispense with "external observers" by introducing the concept of an "event" and locating the predictive content of the formalism in the "preclusion" of certain events. The most straightforward such preclusion rule engenders contradictions which are fatal if one sticks with classical logic, but which can be accommodated by an "anhomomorphic logic" that effectively identifies reality with a Z_2 valued function on the space of possible events.
    Last edited: Jul 15, 2007
  2. jcsd
  3. Jul 15, 2007 #2


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Parallel session talks whose audio is available.

    Here are some of the Loops 07 contributed talks for which both slides and audio are availble. There are too many to list all of them, so I have shown only those which I have heard/watched or (in several cases) have seen recommended.

    If I found a talk especially impressive, particularly if the speaker was presenting results that were new to me, I put the title in boldface. Some good talks I didn't highlight because i was already familiar with the work being presented---in other cases I simply haven't had time to listen to them all.

    Contributed talks

    Benjamin Bahr: Gauge-invariant coherent states for Loop Quantum Gravity · slides (pdf) · audio (mp3)

    The usual complexifier coherent states provide a tool for investigating the semiclassical limit of LQG. However, these states are purely kinematical, i.e. do not satisfy any of the constraints. But if one wants to work on the gauge- or Diff-invariant level, coherent states that satisfy the Gauss- and/or the Diff-constraint are desirable. In this talk I will, as a first step into this direction, describe the projection of the complexifier coherent states on the gauge-invariant Hilbert space and demonstrate their semiclassical properties. This will show that these gauge-invariant coherent states are the appropriate tool for adressing semiclassical issues in the gauge-invariant sector.

    Eugenio Bianchi: Large scale correlations in spinfoam models for quantum gravity · slides (pdf) · audio (mp3)

    In the loop approach to quantum gravity, we study large scale correlations of geometric operators on a semiclassical state peaked on a flat geometry. The correlations found can be compared to the two- and three-point functions computed in perturbative quantum gravity on a flat background. The calculations are done using the Riemannian Barrett-Crane model and involve the boundary amplitude formalism. Perturbative area-Regge-calculus plays a key role as an intermediate step in the comparison between correlations computed from the spinfoam model and from perturbative quantum gravity. The calculation presented is to be considered as a first step towards an effective field theoretical description of the semiclassical regime of loop quantum gravity. The presence of large scale correlation is a key test for spinfoam models for quantum gravity, and the details of the calculation provide constraints and hints for possible improvements of the Barrett-Crane model.

    Wade Cherrington: Numerical Spin Foam Computation of Dual Yang-Mills Theory · slides (pdf) · animation (m4v) · audio (mp3)

    We present a Metropolis algorithm for the computation of dual Yang-Mills theory using spin foam configurations, and describe its implementation for pure Yang-Mills in three dimensions and gauge group SU(2). We report results that are in agreement with conventional computations for a range of coupling constants, and discuss the improvements that will be necessary in treating the weak coupling limit. This work provides evidence that spin foam computations can successfully reproduce known physics while offering a compelling, gauge-invariant picture for the evolution of the physical degrees of freedom. We conclude with an outlook for the application of these spin foam algorithms to matter-coupled spin foam quantum gravity, and to other sectors of the Standard Model.

    Dan Christensen: Computations involving spin networks, spin foams, quantum gravity and lattice gauge theory · slides (pdf) · audio (mp3)

    In this talk I will discuss how computational work involving spin networks and spin foams has lead to further understanding and progress in the areas of quantum gravity and lattice gauge theory. I will discuss positivity of the Barrett-Crane model, the vastly differing behaviour of various versions of it, and new computations of the q-deformed model. Then I will mention the asymptotics of the 10j symbol, the key ingredient of the Barrett-Crane model, and will summarize new results on the graviton propagator. I will end with results showing that spin foam methods can be used as a practical way of performing computations in lattice gauge theory.

    Florian Conrady: Spin foams, gauge-string duality and renormalization · slides (pdf) · audio (mp3)

    In the first part of the talk, we present a recent result on 3d SU(2) lattice Yang-Mills theory, showing that it can be cast in the form of an exact string representation. The derivation starts from the spin foam representation of the lattice gauge theory. We demonstrate that every spin foam can be equivalently described as a worldsheet of strings on a framing of the lattice. Using this correspondence, the expectation value of a Wilson loop is translated into a sum over worldsheets that are bounded by strings along a framing of the loop. In the second part of the talk, we take the worldsheet picture as a motivation to discuss a possible approach to renormalization in SU(2) lattice Yang-Mills theory. The Yang-Mills theory is cast in the form of a lattice BF Yang-Mills theory with both the connection A and a 2-form B as variables. Then, a block spin renormlization is proposed where the block variables are a connection and a B-field on a coarser lattice. We suggest a perturbative scheme for the integration over the UV variables.

    Bianca Dittrich: A new gauge invariant framework for cosmological perturbations · slides (pdf) · audio (mp3)

    Based on the framework of partial and complete observables we introduce a novel perturbative scheme for perturbations around symmetry reduced sectors of general relativity. This scheme allows the calculation of perturbations in a gauge invariant manner to an arbitrary high order. In particular we can consider backreaction effects. Applications to Loop Quantum Cosmology will be discussed.

    Winston J. Fairbairn: Quantization of string-like sources coupled to BF theory: transition amplitudes and topological invariance. · slides (pdf) · audio (mp3)

    Recently, a topological field theory of brane-like sources coupled to BF theory in arbitrary spacetime dimensions was proposed by Baez and Perez. In this talk, I will discuss various aspects of the resulting four dimensional theory. Firstly, I will concentrate on the physical interpretation of the algebraic variables attached to the matter sources by showing how the theory relates to the Polyakov string on a degenerate background and to cosmic string solutions of general relativity. Secondly, I will present a prescription to regularize the physical inner product of the canonical theory and show how the resulting transition amplitudes are dual to evaluations of Feynman diagrams coupled to three-dimensional quantum gravity. Finally, I will discuss the removal of the regulator from the physical inner product by showing that the transition amplitudes of the theory only depend on the equivalence classes of spatial manifolds and embedded string spin networks up to homeomorphisms.

    Iñaki Garay: Quantization of Einstein-Rosen waves coupled with matter · slides (pdf) · audio (mp3)

    Gravity coupled to a massless scalar field with full cylindrical symmetry can be exactly quantized by an extension of the techniques used in the quantization of Einstein-Rosen waves. We obtain the quantum Hamiltonian operator and the unitary evolution operator. This system is a useful testbed to discuss a number of issues in quantum general relativity. In this talk we will study two-point functions and radial wave functions for one-particle states. We will observe some interesting effects such as a large probability to find the particles near the axis or how the null geodesics of an emergent metric appear in the semiclassical limit.

    Igor Khavkine: q-deformed spin foams for Riemannian quantum gravity · slides (pdf) · audio (mp3)

    We numerically study Barrett-Crane models of Riemannian quantum gravity. We have extended the existing numerical techniques to handle q-deformed models and arbitrary space-time triangulations. We present and interpret expectation values of a few selected observables for each model, including a spin-spin correlation function which gives insight into the behaviour of the models. We find the surprising result that, as the deformation parameter q goes to 1 through roots of unity, the limit is discontinuous. Joint work with Dan Christensen.

    Garrett Lisi: Deferential Geometry · slides (pdf) · audio (mp3)

    The message from gravity is clear: our universe is geometric. But gravity does not exist alone -- it interacts with the whole zoo of standard model fields. A unification of these fields requires the gravitational connection to be absorbed as part of a larger connection, along with the intimately intertwined Higgs field and frame of GR. By extending this connection further, one generation of fermions may be derived as BRST ghosts, and the dynamics described by a modified BF theory. This description meshes well with recent approaches quantizing gravity perturbatively around a topological theory. But where could such a big, messy connection come from? As it turns out, precisely this connection matches the geometry of the largest exceptional Lie group -- giving exactly three complete generations of fermions, linked by triality! In this fully unified picture, gravity and all fields of the standard model are described by the pure geometry of a group manifold, with no strings attached.

    Seth Major: An alternative to the loop algebra of q-Quantum Gravity? · slides (pdf) · audio (mp3)

    A long-standing proposal for a deformed loop representation of q-Quantum Gravity is addressed.

    Merced Montesinos: Cartan's equations define a topological field theory · slides (pdf) · audio (mp3)

    Cartan's first and second structure equations together with first and second Bianchi identities can be interpreted as equations of motion for the tetrad, the connection, the torsion, and the curvature. In this sense these equations define by themselves a field theory. Restricting the analysis to 4-dimensional spacetime manifolds (keeping gravity in mind), it is possible to give an action principle of the BF type from which these equations of motion are obtained. The action turns out to be equivalent to a linear combination of the Nieh-Yan, Pontrjagin, and Euler classes, and so the field theory defined by the action is topological. The inclusion of gravity is also discussed.

    Hanno Sahlmann: Exploring the diffomorphism invariant Hilbert space · slides (pdf) · audio (mp3)

    something about diffeomorphism invariant functionals for scalar- and gauge-fields

    Yuya Sasai: Braided quantum field theories and their symmetries · slides (pdf) · audio (mp3)

    Braided quantum field theories proposed by Oeckl can provide a framework for defining quantum field theories with Hopf algebra symmetries. In quantum field theories, symmetries lead to non-perturbative relations among correlation functions, which are generally called Ward-Takahashi identities. We discuss Hopf algebra symmetries and Ward-Takahashi identities in braided quantum field theories. We give the four algebraic conditions between Hopf algebra symmetries and braided quantum field theories, which are required for Ward-Takahashi identities to hold. As concrete examples, we apply our discussions to the Poincare symmetries of two examples of noncommutative field theories. One is the effective quantum field theory of three-dimensional quantum gravity coupled with spinless particles given by Freidel and Livine, and the other is noncommutative field theory on Moyal plane. We also comment on quantum field theory on kappa-Minkowski spacetime.

    Joshua Willis: Recent Work on Computing Lorentzian Spin Foams · slides (pdf) · audio (mp3)

    I will review recent work on computing Lorentzian 10J symbols and related functions.

    Last modified 13 July 2007.
    Just as a reminder, here is a small sample of talks which we still don't have both slides and audio for:

    James Ryan: Aspects of Group Field Theory · audio (mp3)

    Frederic P. Schuller: Area metric gravity · audio (mp3)

    The Lorentzian spacetime metric is replaced by an area metric which naturally emerges as a generalized geometry from the canonical quantization of gauge theories, string theory and gravity. Employing the area metric curvature scalar, the gravitational Einstein-Hilbert action is re-interpreted as dynamics for an area metric. Without the need for dark energy or fine-tuning, area metric cosmology explains the observed small acceleration of the late Universe.

    Parampreet Singh: Exactly Solvable LQC: New Insights on some old questions · slides (pdf)

    Under very mild assumptions we are led to an exactly solvable model in LQC that can be rigorously compared to the Wheeler-DeWitt theory. We show that LQC and Wheeler-DeWitt can be constructed from the solutions of a 1+1 dimensional Klein-Gordon theory. Like any exactly solvable model the non-triviality lies in the mapping between the simple common description and the physical theories. Working with a complete set of solutions we provide an explicit formula for the evolution of the volume operator -- the only non-trivial Dirac observable in the theory and show that: (1) Quantum Bounce is not restricted to semi-classical states in the simple models studied so far, (2) There is a precise sense in which Wheeler-DeWitt theory is not a continuum limit of LQC and (3) LQC is a fundamentally discrete theory.

    Johannes Tambornino: Backreactions in cosmological perturbation theory · audio (mp3)

    I will talk about a recently proposed perturbation scheme, that allows to calculate cosmological perturbations up to arbitrary order. This framework is based on the concept of relational observables and approximates observables of full GR around some symmetry reduced sector. I will compare this framework to the standard theory of cosmological perturbations and will illustrate how to calculate backreaction effects using Bianchi-I as a simple cosmological model.

    Kevin Vandersloot: Dynamics of Loop Quantum Schwarzschild Interior · slides (pdf)

    We discuss dynamics of the Schwarzschild interior using an effective semi-classical description of the loop quantization. We will consider the effects of an improved loop quantization using techniques from loop quantum cosmology.
    Last edited: Jul 15, 2007
  4. Jul 15, 2007 #3


    User Avatar

    I wish that I could have been there. Maybe, I would have been able to get answers to improve my blog.
    Here is another question from
    QUANTUM GRAPHITY: a background independent condensed
    matter model of emergent space
    Fotini Markopoulou
    Perimeter Institute
    T. Konopka, FM & L.Smolin, hep-th/0611197
    also with: O. Dreyer, S. Severini
    What is the meaning of the last slide?
    Theoretical/ideal -> Actual ..... OR .... Empty space -> space with matter content.???
    OR ??

    Second presentation
    Tomasz Konopka: Statistical Properties of Quantum Graphity

    I did get some answers.
    Remember …. The Expanding universe …. Simplest solution …add an additional unit.
    Quantum Evolution in an Expanding Hilbert Space
    Isabeau Prémont-Schwarz
    Perimeter Institute and Univ. of Waterloo
    In collaboration with: David Kribs and Fotini
    Perimeter Institute seems to have had a lot of presenters.
  5. Jul 18, 2007 #4


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Bojowald's audio is up


    Bojowald's talk is especially timely because of the current controversy about the Bounce,
    his discovery of a "forgetfulness" or indeterminacy that limits knowledge of the prior contracting phase in the LQC model,
    as described in his article in the current issue of Nature Physics.

    the Loops 07 website just put up the audio for Bojowald's talk
    Loop quantum gravity and effective theory · slides (pdf) · audio (mp3)

    "Several models have been defined to study the broad framework of loop quantum gravity. We present a common perspective for cosmology where configurations are close to being isotropic. Effective techniques are then used to illustrate important features of the semiclassical limit and to show examples of correction terms to Einstein's equation. Applications include corrections to the Newton potential and to cosmological perturbation equations relevant for the CMB power spectrum."



    also, on the contributed talks page I see that the audio is now posted for Tim Koslowski's talk
    Reduction of a Quantum Theory - Cosmological Degrees of Freedom in Loop Quantum Gravity · slides (pdf) · audio (mp3)

    "A procedure to explicitly construct a reduced quantum system in a full quantum theory without classical reduction and requantization is presented and applied to LQG to obtain a cosmological sector."
    Last edited: Jul 18, 2007
  6. Jul 20, 2007 #5


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

  7. Jul 26, 2007 #6


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

  8. Aug 7, 2007 #7


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Part 3 of Alejandro's report on Loops '07 is out on his blog:


    He's done a nice job of covering the discussions that Carlo Rovelli led, where they discussed questions collected from the participants
    and also different people in the community informally shared their visions of the future---with some exchanges evoking great hilarity.

    It's good to have reporting like this that gives an idea of what else besides the individual presentations---and gives personal reactions to the talks.

    Abundant thanks to Alejandro and his blog "Realtity Conditions"!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook